(a) Field of the Invention
The present invention relates to a negative electrode active material for a lithium-based secondary battery and a method of preparing the same and, more particularly, to a negative electrode active material serving to fabricate a lithium-based secondary battery having a high capacity.
(b) Description of the Related Art
In recent years, the development of miniaturized portable electronics provokes needs for the secondary battery having a high capacity as well as a light weight. From the viewpoint of the capacity improvement per unit weight, the lithium-based secondary battery is preferably adopted because lithium has a high standard potential as well as a low electrochemical equivalent weight.
FIG. 2 is a schematic cross sectional view showing the general structure of the lithium-based secondary battery. As shown in FIG. 2, the lithium-based secondary battery includes a positive electrode plate 40 having a collector 1 made of nickel and an active material layer 10 coated on the collector 1, a negative electrode plate 45 having a collector 1′ made of copper and an active material layer 30 coated on the collector 1′, and a separator 25 interposed between the positive and negative electrode plates 40 and 45. The positive and negative electrode plates 40 and 45 essentially form an electrode plate assembly together with the separator 25. The electrode plate assembly is inserted into an opening portion of a battery case 5 internally surrounded with a gasket 20 while receiving an electrolyte 15 therein. The opening portion of the battery case 5 is covered by a cap 35.
Lithium-containing transition metal oxides such as LiCoO2, LiNiO2, LiMn2O4 and LiNixCo1-xOy are preferably selected for the positive electrode active materials in conjunction with a polyethylene-based porous film for the separator.
As for the negative electrode active materials, lithium metal is attractive because it has a light weight and high capacity per unit weight to thereby output high voltage in the battery use. However, the use of lithium metal for the negative electrode material reveals serious defects in a cycle life and stability of the battery because the lithium metal is highly reactive with the electrolytic solvent and easily forms needle dentrites during cycling, causing destruction of the separator and a short circuit. In order to avoid the defects, lithium alloys are employed as the negative electrode active material instead of the lithium metal but yet reveals similar problems.
Alternatively, carbon materials, which can reversibly accept and donate significant amounts of lithium without affecting their mechanical and electrical properties, are proposed for the negative electrode active material.
The carbon materials adapted for the battery use can be divided into an amorphous carbon material and a crystalline carbon material.
When the amorphous carbon material is used for the negative electrode, the charge and discharge capacity is high but irreversible reactions are frequently generated and flatness in output voltage is poor compared to the crystalline carbon material. On the contrary, when the crystalline carbon material is used for the negative electrode, flatness in output voltage as well as the cycle life is good and the discharge/charge efficiency is high. But, the charge and discharge capacity is poor compared to the amorphous carbon material. Among them, the crystalline carbon material, i.e., the graphite-like carbon material has been preferably adopted for the negative electrode active material and trial has been made to increase its charge and discharge capacity.
Meanwhile, even the graphite-like carbon material has only a partial crystalline structure or a complicated structure of crystalline portions and amorphous portions.
Therefore, U.S. Pat. No. 5,436,092 discloses a technique of controlling a ratio of crystallinity by separating the X-ray diffraction peaks at the (002) plane and the (10) plane, being a combination of (100) plane and (101), into a crystalline component and an amorphous component.
Furthermore, U.S. Pat. No. 5,340,670 discloses a technique of controlling the intensity ratio I(101)/I(100) of the X-ray diffraction peak at the (100) plane to the X-ray diffraction peak at the (101) plane.
In the meantime, U.S. Pat. No. 5,344,724 discloses a technique of controlling the graphitization degree of the graphite-like carbon material.
However, the aforementioned techniques does not satisfactorily improve the charge and discharge capacity of the graphite-like carbon material.